Our Sun Is Moving More Slowly Than Thought

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The sun is zipping through interstellar space more slowly than
once thought, suggesting the giant shock wave long suspected of
existing in front of the sun is not actually there, researchers
say.

These new findings may influence what scientists know about
high-energy cosmic rays that can endanger astronauts, they added.

The sun and its planets are cocooned within a bubble of charged
particles and magnetic fields known as
the heliosphere. The edge of the heliosphere, where it
collides with interstellar gas and dust, is called the
heliopause, and marks the outer limit of the solar system.

For about a quarter century, researchers had thought
the sun was moving fast enough in space for our heliosphere
to generate a shock wave known as a bow shock as it plowed
through interstellar matter.

"The sonic boom made by a jet breaking the sound barrier is an
earthly example of a bow shock," said study lead author Dave
McComas, a space scientist at the Southwest Research Institute in
San Antonio, Texas. "As the jet reaches supersonic speeds, the
air ahead of it can't get out of the way fast enough. Once the
aircraft hits the speed of sound, the interaction changes
instantaneously, resulting in a shock wave."

Astronomers have seen these bow shocks of plasma before in space,
with counterparts of the sun's heliosphere around distant stars
known as astrospheres. [ Amazing
Sun Photos from Space ]

"By studying our heliosphere in detail, we can learn more about
the whole picture of how astrospheres work around stars," McComas
said.

New observations now show that our sun is traveling slower than
before thought. As such, it moves too slowly to generate a bow
shock.

"We've now discovered our sun doesn't have a bow shock — it's
surprising, and slightly shocking, and much work needs to be
redone now," McComas told SPACE.com. "The astronomical community
spent the last two or three decades studying something that
doesn't exist."

Data from NASA's
Interstellar Boundary Explorer (IBEX), a small spacecraft
remotely imaging the nature of particle interactions at the edge
of our solar system, reveals our sun is zipping through the local
interstellar cloud at about 52,000 miles per hour (83,700 kph).
This is roughly 7,000 mph (11,250 kph) slower than previously
thought — a dip in speed that by itself would drop the pressure
the heliosphere experiences by about one-quarter, enough to keep
a bow shock from developing.

"The instruments we have to directly observe our motion with
respect to the local interstellar medium are more modern, for
more precise measurements," McComas explained.

IBEX data and earlier observations from
NASA's two Voyager probes also show that the interstellar
medium's magnetic field is stronger than previously believed.
This means the sun would have had to move even faster than
previously thought in order to generate a bow shock. Altogether,
this suggests that a bow shock upstream of the sun is highly
unlikely.

"While bow shocks certainly exist ahead of many other stars,
we're finding that our sun's interaction doesn't reach the
critical threshold to form a shock, so a wave is a more accurate
depiction of what's happening ahead of our heliosphere, much like
the wave made by the bow of a boat as it glides through the
water," said McComas, principal investigator of the IBEX mission.

Heliosphere unmasked

A better understanding of the heliosphere could shed light on how
it protects us against dangerous high-energy cosmic rays emerging
from deep in outer space.

"The heliosphere screens out about 90 percent of cosmic rays,"
McComas said. "If not for the heliosphere, cosmic rays could
affect things like human space travel and even potentially things
on Earth. It's interesting to consider that maybe astrospheres
are needed as a prerequisite for life around stars, to protect
systems from galactic cosmic radiation."

The fact that our solar system lacks a bow shock could actually
mean we are slightly more protected from
cosmic rays than before thought.

"The slower speed the sun is traveling at with respect to the
local interstellar medium means that at the moment, there is less
compression of the heliosphere, so there's a larger region to
deflect cosmic rays," McComas said. "However, in the distant
past, as the sun goes in its orbit around the galaxy, it surely
went through dense clouds, so maybe the heliosphere got squeezed
then, and we were more affected by galactic radiation. Surely
that will happen again in the distant future."

The scientists detailed their findings online May 10 in the
journal Science.

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